The present invention relates to an organic electroluminescence (EL) device and a liquid crystal display using the organic EL device as a backlight.
A liquid crystal display is extensively used as a display in, for example, portable devices. With an increasing demand for smaller, lighter, and less power consuming portable devices, thinner liquid crystal displays having low power consumption are becoming necessary.
Reflection type liquid crystal displays are being used to reduce the power consumption, but the image quality thereof is not sufficient. A transmissive (including semi-transmissive) liquid crystal display using a backlight excels in achieving a sufficient image quality. Recently, the use of a light-emitting element, such as an organic EL element, for a backlight has been proposed and has been put into actual application.
Furthermore, organic EL elements have gained attention as being the elements for the next generation of displays following liquid crystal displays. In general, an organic EL element is configured by forming a transparent electrode (anode) made of indium-tin-oxide (ITO) on a glass substrate, forming organic layers including a light-emitting layer on the transparent electrode, and then superimposing an opaque cathode on the organic layer. Such an organic EL element has a bottom-emission structure in which light emitted from the light-emitting layer exits from the glass substrate side.
The organic EL element performs carrier injection type light emission. Thus, the electric energy that is not converted to EL light emission is converted to Joule heat. The Joule heat increases the temperature of the device. This results in problems such as decomposition of the organic compounds constituting the organic EL element, decrease in brightness, and shortening of lifetime. If the organic EL has a light-emission efficiency of 100%, the generation of the Joule heat would be zero. However, such situation is impossible in reality. Thus, an organic EL element that effectively dissipates the heat generated during light-emission of the organic EL element has been proposed in the prior art. Such an organic EL element is disclosed in for example, Japanese Laid-Open Patent Publication No. 4-129194, Japanese Laid-Open Patent Publication No. 8-124679, and Japanese Laid-Open Patent Publication No. 2002-117973.
Japanese Laid-Open Patent Publication No. 4-129194 proposes forming the organic EL element on a substrate having high thermal conductivity. Further, to employ the organic EL element in a display, a configuration is proposed for arranging the organic EL element in contact with a substrate having high thermal conductivity between thin electrical insulation layers.
Japanese Laid-Open Patent Publication No. 8-124679 proposes arranging an organic EL device on a metal substrate with a metal thin film arranged therebetween. The metal thin film has high thermal conductivity.
In Japanese Laid-Open Patent Publication No. 2002-117973, an element region is formed by superimposing a first electrode, an organic compound layer including a light-emitting layer, and a second electrode layer on a glass substrate. Further, a protective film composed of heterocyclic compound polymer is formed thereon so as to cover the entire element region of the substrate. The protective film has sufficient heat resistance for an organic EL element, and has relatively high thermal conductivity. Thus, the Joule heat generated by driving the organic EL element is dissipated to the protective film and radiated from the surface of the protective film. Furthermore, the above publication also states that the use of a forced cooling means, such as Peltier cooling or fan cooling, together with the protective film facilitates the radiation of heat from the surface of the protective film.
In the configurations described in Japanese Laid-Open Patent Publication No. 4-129194 and Japanese Laid-Open Patent Publication No. 8-124679, heat radiation is enhanced by arranging an organic EL element or an organic EL device directly or by way of a thin film on a substrate having high thermal conductivity. Hence, the organic EL element or organic EL device cannot be cooled to a temperature lower than ambient temperature. Furthermore, Japanese Laid-Open Patent Publication No. 2002-117973 discloses the use of a forced cooling means, such as Peltier cooling, under the assumption that an organic EL element is formed on a glass substrate. However, when the forced cooling means is used, the characteristic of the organic EL device, which is a device that is thin, is sacrificed.